The Rspondin protein family is conserved among vertebrates and consists of the four related members Rspondin1-4 (Rspo1-4) (Chen et al., 2002, Mol. Biol. Rep. 29, 287-292, who called Rspo3 hPWTSR; Kamata et al., 2004, Biochim. Biophys. Acta.1676, 51-62; Kazanskaya et al., 2004, Dev. Cell 7, 525-534; Kim et al., 2005, Science 309, 1256-1259; Kim et al., 2006, Cell Cycle 5, 23-26; Nam et al., 2006, J. Biol. Chem. 281, 13247-13257). Human Rspo1-4 were also described as Stem Cell Growth Factor Like Polypeptides, which are able to promote proliferation of hematopoietic stem cells (WO 01/77169; WO 01/07611). They were also designated as Futrin1-4 and identified as modulators of the Wnt signalling pathway (WO 2005/040418). The content of these documents is herein incorporated by reference and the amino acid and nucleic sequences of Rspondins 1-4 disclosed therein are specifically included herein.
The Rspo genes encode secreted proteins which can activate Wnt/b-catenin signalling, and Rspo2 promotes myogenesis via the Wnt/b-catenin signalling pathway in Xenopus (Kazanskaya et al., 2004, Dev. Cell 7, 525-534). Rspondin genes are widely coexpressed with Wnt genes in many regions during embryonic development, and Rspondin expression is positively regulated by Wnt signals (Kamata et al., 2004, Biochim. Biophys. Acta.1676, 51-62; Kazanskaya et al., 2004, Dev. Cell 7, 525-534). Furthermore, it was reported that secreted human Rspo1 promotes proliferation of intestinal epithelium through stabilizing of b-catenin (Kim et al., 2005 Science 309, 1256-9). Mutation of mouse Rspo3 results in embryonic lethality and induces severe defects in the development of the placenta (Aoki et al., Dev Biol. 2007 301(1):218-26). However, no effect on blood vessel development was reported in this mutant model and, in contrast to the results disclosed herein, the embryos appeared to show no sign of haemorrhage, therefore there was no suggestion before the present invention that Rspondin, in particular Rspondin 2 or 3, played a significant role in angiogenesis and/or vasculogenesis.
Angiogenesis is likely to be regulated by polypeptide growth factors. Several polypeptides with in vitro endothelial cell growth promoting activity have been identified. Examples include acidic and basic fibroblastic growth factor, VEGF and placental growth factor.
VEGF is a key factor in vasculogenesis and angiogenesis and its signalling pathway an important target for pharmacological intervention (Ferrara 2005, Oncology 3:11-6; Rosen 2005, Oncologist 10:382-91).